1. Field of the Invention
This invention relates to improvements in vibrating tine resonators useful as force transducers or the like and, more particularly, to resonators having an improved tine system and to structure and methods for operating the resonator in a dynamic mode including torsional and normal components.
2. Description of the Prior Art
Resonators with tines or beams have been used as frequency determining elements in electronic oscillator circuits. A characteristic of such devices is that the resonant frequency at which they vibrate varies in response to an applied force such as an axial force applied to the tines. This characteristic has been exploited by using resonating tine devices as transducers to measure parameters including acceleration, force, temperature, pressure and weight.
In a typical transducer implementation, the resonator may be made of a piezoelectric material such as crystalline quartz, and electrodes applied to the tines couple the resonator to an oscillator feedback circuit. Energy applied by the electrodes excites the resonator to vibrate at a resonant frequency. Frequency variations indicating changes in the measured parameter are detected to provide an output for instrumentation, control or the like. Alternatively, the resonator may be made of silicon and driven electrostatically or electromagnetically.
To provide for balancing of forces within the resonating device, one common configuration for a resonator is a double ended tuning fork (DETF) having a pair of spaced and parallel tines or beams fixed at opposite ends to integral end portions. A slot separates the beams, and a crotch is defined at each end portion where the beams terminate. Each tine is mechanically coupled to the other by forces communicated between the ends of the tines through the end portions. The result is that the tines vibrate in sympathetic motion at the same frequency, but in opposite, out-of-phase directions. Resonators with more than two tines, and many different structural features such as shoulders, notches, added masses and the like have been proposed to increase the efficiency of vibrating tine structures.
Vibrating tine resonators are typically made from a flat sheet or web of crystal material, often piezoelectric quartz. When quartz is used, the electrodes are usually bonded to the tines to apply electrical potentials resulting in piezoelectric forces in the quartz material. If a different material such as silicon is used, electromagnetic or electrostatic electrodes may be used. Normally, the electrodes are arrayed to cause the tines to vibrate in the plane of the sheet material from which the device is formed.
U.S. Pat. No. 4,469,979 discloses a DETF resonator in which electrodes are disposed on all four surfaces of each tine. The electrodes are deployed to optimize a desired fundamental or harmonic flexure mode. U.S. Pat. No. 4,773,493 discloses DETF resonators with rotational masses coupled to nodal points along the length of the tines to influence the tines to resonate at the intended measurement frequency.
Resonators employed in the past are subject to problems. When the tines vibrate in the plane of the stock in air or another gaseous atmosphere, gas is forced from between adjacent tines as they move toward one another and then is pulled or sucked into the space between the tines as they separate from one another. This effect, called gas pumping or air pumping, results in loss of energy, reducing the Q and the efficiency of the resonator. Q is the ratio of energy stored in the resonator in each cycle of vibration to the energy lost per cycle. Because the value of the ratio Q is directly proportional to the gain and thus the sensitivity of the crystal controlled oscillator, the reduction of Q due to gas pumping is undesirable. Gas pumping can also decrease the linearity of the system because of inconsistent resistance to tine motion. Although the disadvantageous effects of gas pumping can be avoided by operating the device in a vacuum, this may not be a practical or cost effective solution.
In the case of quartz resonators, the array of electrodes used to piezoelectrically drive the tines in the plane of the stock usually includes electrodes on the sides of the tines. While it is relatively easier to form electrodes on the outwardly facing surfaces of the stock, it is more difficult and expensive to form electrodes upon the side surfaces. One reason for the difficulty is the limited space between adjacent tines.
It has been proposed to use three tines in a resonator in an attempt to decrease the loss of energy resulting from coupling unbalanced reaction forces to the support for the resonator, and in an attempt to increase to maximum axial force measurement capability. A three tine device has disadvantages including difficulty in manufacture due to critical beam mass balancing requirements.